Organic light emitting diode (OLED) is a flat panel display technology, which has great prospects for development. It possesses not only extremely excellent display performance but also properties of self-illumination, simple structure, being ultra thin, fast response speed, wide view angle, low power consumption and capability of realizing flexible displaying, and is therefore considered as a “dream display”. It has been favored by all large display makers and has become the main selection of the third generation displays.
The OLED display generally comprises a substrate, an anode located on the substrate, an organic emitting layer located on the anode, an electron transport layer located on the organic emitting layer, and a cathode located on the electron transport layer. During operation, holes from the anode and electrons from the cathode are injected into the organic emitting layer. The electrons and holes are combined to generate excited electron-hole pairs, and the excited electron-hole pairs are converted from an excited state to a ground state for achieving illumination.
The OLED can be categorized as passive matrix OLED (PMOLED) and active matrix OLED (AMOLED) according to driving type. The power consumption of the PMOLED is high, and thus, it hinders the application in large-scale display devices. Besides, in PMOLED, the aperture ratio is decreased along with the amount of wirings increased. Therefore, the PMOLED is generally applied for the small-scale display devices. The lighting efficiency of the AMOLED is high, and therefore, it is generally utilized for the large-scale display devices of high resolution.
On the other hand, the AMOLEDs can be categorized into bottom emitting AMOLED display devices and top emitting AMOLED display devices according to the emitting direction of the light from the organic emitting layer.
Referring to FIG. 1, a sectional view of a prior art bottom emitting AMOLED display device is provided. As shown in FIG. 1, the AMOLED display device comprises a first substrate 10 and a second substrate 20, which are separated and oppositely face each other. A plurality of thin film transistors T and a plurality of first electrodes 31 are formed on an inner surface of the first substrate 10, wherein each of the first electrodes 31 is connected to one of the thin film transistors T, and an organic layer 32 is formed on the first electrodes 31 and the thin film transistors T, and second electrodes 33 are formed on the organic layer 32. The organic layer 32 emits three colors of light in each pixel P, including red light R, green light G and blue light B. The AMOLED display device further comprises: a dryer 21 formed on an inner surface of the second substrate 20 to remove moisture and air that possibly invade into a space between the first and second substrates 10, 20. Seal 12 is arranged between the first and second substrates 10, 20 and surrounds the first and second electrodes 31, 33, the organic layer 32, and the thin film transistors T for protecting these elements from external moisture and air.
In the bottom emitting AMOLED display device shown in FIG. 1, the light emitting therefrom passes through the bottom where the thin film transistors are formed, and this lowers the aperture ratio as compared with the top emitting AMOLED display device. On the other hands, the top emitting AMOLED display device possesses a high aperture ratio but the cathode is generally built on the organic layer so that choice of the material for manufacturing the cathode is restricted. Thus, the transmittance is restricted and display effect is degraded.